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Geography of Hazards Final Review Notes.docx

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Geography 2152F/G
Mark Moscicki

Geo Of HazardsLecture 7EarthquakesEARTHQUAKESResult from the rupture of rocks along a faultEnergy from an earthquake is released in the form of seismic wavesThey are mapped according to the epicenter the focus is located directly below the epicenterThey are measured by seismograph and compared by magnitudeEarthquake MagnitudeThe magnitude of an earthquake can be expressed as a number to one decimal placeThis type of measurement was first developed by Richter in 1935 we do not currently use this scale despite people in the media referring to itThe Richter Scale was a measure of the strength of a wave at a distance of 100km from the epicenterSince then more accurate methods have been developed richter scale no longer in useThe Moment Magnitude scale MThe scale is determined byoThe area ruptured impactedbreakage along a faultoThe amount of movement along the faultoThe elasticity of the crust at the focusSimilar to Richter scale it is logarithmicEx A M6 earthquake has 10 times the amount of ground motion as an M5 earthquakeexam question along these linesMagnitude and Frequency of EarthquakesExcept for very large earthquakes the magnitude on the Moment Magnitude Scale is similar to the Richter scale anything over 8 is considered catastrophic strengthStrongest earthquake everM95 in Chile 1960In Canada M81 in BC 1949News worth earthquakes are typically 45 in north AmericaThere are only a few M9 earthquakes each centuryEarthquake IntensityThe Modified Mercalli Intensity scalequalitative scale based on damage to structure and the affect on peopleNote elongated nature of the fault and resultant waves see slide 6It is based on 12 categories IXIIMaps often produced showing the differences in modified mercalli intensities over broad areaEarthquake ProcessesMost common at or near plate boundariesMotion at plate boundaries is not usually smooth or constantFriction along plate boundaries exerts a force stress on the rocks exerting strain or deformationWhen the stress exceeds the strength of the rocks there is a sudden movement along the faultoNote most earthquakes are probably only around 30 seconds in durationThe movement or rupture starts at the focus and propagates in all directions called seismic wavesThus faults are considered seismic sourcesIdentifying faults is necessary for risk evaluation of a given areaNot all faults reach the Earths surface Blind faults are located below the surfaceNew technology GPR groundpenetrating radar is allowing us to pinpoint where the blind faults may beFault TypesThere are two basic types of geologic faults distinguished by the direction of the displacement of rock or sedimentStrikeslip faultsoDisplacements are horizontal basically neither rock risessinks just lateral movement along the faultoBest example The San Andreas faultDipSlip faults see slide 15oDisplacements are vertical often considered more dangerousviolentoThree types reverse fault thrust fault and normal faultoComprised of two walls on an incline defined by miners asFootwall where miners placed their feetHangingwall where miners placed their lanternsoReverse faultThe hangingwall has moved upwards relative to the footwall inclined at an angle steeper than 45 degreesoThrust faultThese are similar to reverse faults except the angle is 45 degrees or lessoNormal faultThe hanging wall has moved downward relative to the footwallFault ActivityOne of 3 categoriesoActiveMovement during past 11600 yearsoPotentially ActiveMovement during past 26 million yearsoInactiveNo movement during past 26 million yearsTectonic CreepThe slow movement of rock or sediment along a fracture caused by stressalso referred to as fault creepThis can damage roads and building foundations ie movement of a few cm per decadeAlong the faults periodic sudden displacement producing earthquakes can also occurSeismic WavesSome seismic waves generated by fault rupture travel within the body of the Earth and others travel along the surfaceBody waves these include P and S wavesP waves these are primary or compressional wavesoMove fast pushpull motion and can travel through solids or liquids mantleSwaves secondary or shear wavesoMove more slowly in an upanddown motions and can only travel through solidsSurface wavesoSeismic waves that form when P and S waves reach Earths surface and the move along itoThese waves are slower than body wavesoThey are responsible for damage near the epicenterEarthquake ShakingFactors that determine the shaking people experienceoMagnitude distance to epicenter focal depth how deep below surface was the rupture direction of rupture is the fault running northsouth eastwestoLocal soil andor rock typehardness moisture contentoLocal engineering and construction practicesretrofittingdesignSeismographs record the arrival of waves to a recording station because P waves travel faster than S waves they always appear first on seismogramEarthquake shaking decreases with distance from the epicenterDistance to the Epicentre will have exam Q similar to thisThe difference between the arrival times of the first P and S waves at different locations determines the distance to the epicenterThe distance to the epicenter is calculated at 3 different seismic stationsA circle with a radius equal to that distance is drawn around the stationLocating an EarthquakeThe epicenter is located where the circles intersect this is called triangulationFocal DepthSeismic waves lose some of their energy before they reach the surfaceThe greater the focal depth the less intense the shaking at the surfaceThis loss of energy is referred to as attenuationDirection of RuptureEarthquake energy is focused in the direction of rupture reasoning behind the oval shape of the wave strengthThis is known as directivity and contributes to increased shakingRadiated waves are sometimes stronger in one direction along the faultProximity to a fault therefore isnot necessarily a DIRECT indicator of shaking you will experienceLocal Soil and Rock TypesThe local geology influences the amount of ground motionDense homogeneous crust bedrock can transmit earthquake energy quicklySeismic energy slows down in areas with heterogeneous folded faulted crust
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